Pressure oscillations occurring in the combustors of modern gas turbines, so-called combustor pulsations or combustion pulsations, often also simply called pulsations, provide important indications of the quality of the combustion, especially when employing premix burner technology. Under unfavorable conditions, the combustor pulsations may reach amplitudes at which the mechanical integrity of gas turbine components is at risk. This means that a permanent monitoring of combustor pressure oscillations basically is now indispensable. Because of the high temperatures, a direct detection of occurring pressure oscillations requires high-temperature-resistant pressure sensors, which on the one hand are very expensive, and on the other hand are confronted with usage conditions that are so extreme that a significant probability of failure exists during continuous operation. It is also known that the sensor characteristic of such sensors is temperature-dependent, which also makes the quantification of the measured pressure oscillations harder or allows it only with limited accuracy. From DE 10 2007 035 013 A1 a combustion monitoring system comprising a probe with one pressure transmitter is known. FIG. 2 of DE 10 2007 035 013 A1 illustrates the installation of that probe with its tip and its pressure transmitter located inside a flue socket.
U.S. Pat. No. 6,550,336 B2 describes a combustion monitoring system comprising sensor that is set back from the combustor wall a distance by means of a semi-infinite tube in order to not expose the sensor to the high temperatures of the combustion. In these
so-called long-line probes, the actual measuring point within the gas turbine combustor is connected by means of a line, basically by means of a small tube, with a pressure transmitter positioned outside of the combustor. This concept exposes the pressure transmitter to lower temperatures due to distance between the combustor chamber and the pressure transmitter. For this reason, substantially cheaper pressure transmitters or microphones, whose useful life and measuring accuracy is not limited by extreme usage conditions, can be used.
In such a configuration, it is important to ensure an echo-free termination of the measuring line formed in this manner, and, if possible, to also avoid any type of reflections within the measuring line. The termination of the measuring tube with a semi-infinite tube is known. This is realized with a line having a long length, which line is connected on a first end with the end of the measuring tube facing away from the measuring point. With sufficient length, the pressure oscillations are attenuated inside the semi-infinite tube as a result of internal dissipation in such a way that no amplitude worth mentioning is reflected anymore at the second end of the semi-infinite tube.
At higher amplitudes the recorded acoustic signal is distorted and does not contain the full range of frequencies. The reason is the very long distance between the combustor area and the pressure transmitter, which is located outside the engine casing. Additionally, temperature and pressure effects further add or reduce frequency depending damping of the signal. A complicated calibration concept would therefore be needed to calibrate the probe for practical use.
Basically, both currently available approaches do not allow for a accurate and redundant pulsation measurement: Multiple pressure transmitters outside the casing will not capture the full frequency range of interest because they are not close enough to the combustor and the long line probe with pressure transmitters mounted on the back and suffers from frequency dependent damping and signal distortion.
It is an object of the claimed invention to overcome the disadvantages of the prior art and provide redundant probe that captures the full frequency range of interest without significant signal distortion and damping.